Cyclic nucleotide phosphodiesterases (PDEs) are enzymes that regulate the cellular levels of the second messengers, i.e., cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), by controlling their rates of degradation. PDEs are a superfamily of enzymes encoded by 21 genes and subdivided into 11 distinct families according to structural and functional properties. The PDE enzymes selectively catalyze the hydrolysis of the 3′-ester bond of cAMP and/or cGMP, forming the inactive 5′-monophosphate. On the basis of substrate specificity, the PDE families can be further classified into three groups: i) the cAMP-PDEs (PDE4, PDE7 and PDE8), ii) the cGMP-PDEs (PDE5, PDE6 and PDE9), and iii) the dual-substrate PDEs (PDE1, PDE2, PDE3, PDE10 and PDE11).
cAMP and cGMP are involved in the regulation of virtually every physiological process such as pro-inflammatory mediator production and action, ion channel function, muscle relaxation, learning and memory formation, differentiation, apoptosis, lipogenesis, glycogenolysis and gluconeogenesis. Especially, in neurons, these second messengers have an important role in the regulation of synaptic transmission as well as in neuronal differentiation and survival (Non-Patent Document 1). Regulation of these processes by cAMP and cGMP are accompanied by activation of protein kinase A (PKA) and protein kinase G (PKG), which in turn phosphorylate a variety of substrates, including transcription factors, ion channels and receptors that regulate a variety of physiological processes. Intracellular cAMP and cGMP concentrations seem to be temporally, spatially, and functionally compartmentalized by regulation of adenylate and guanylate cyclases in response to extracellular signaling and their degradation by PDEs (Non-Patent Document 2). PDEs provide the only means of degrading the cyclic nucleotides cAMP and cGMP in cells, thus PDEs play an essential role in cyclic nucleotide signaling. Thereby, PDEs could be promising targets for various therapeutic drugs.
Phosphodiesterase 2A (PDE2A) is a dual substrate enzyme that hydrolyzes both cAMP and cGMP. It is organized into four domains, i.e., N-terminus, GAF-A, GAF-B, and catalytic domains, and functions as a homodimer. PDE2A catalytic activity is allosterically stimulated by cGMP binding. GAF-B domain binds with a high affinity and a high selectivity to cGMP. A conformational change is caused by the cGMP binding in the PDE2A homodimer which causes a severalfold or more increase in the catalytic activity of the enzyme (Non-Patent Document 3-6). In contrast, there are as yet no known in vivo examples that cAMP stimulates PDE2A catalytic activity, even though it can also bind to the GAF-B domain with a 30-100-fold lower affinity than cGMP (Non-Patent Documents 7 and 8). PDE2A activity may become functionally significant under conditions in which cellular cGMP concentrations are elevated, which shows a physiological role for GAF domain-regulation of the enzyme.
PDE2A is weakly expressed in a wide variety of tissues and highly in the brain. The activity and protein were originally purified from heart, liver, adrenal gland, platelets, endothelial cells, and macrophages (Non-Patent Documents 9-14). In the brain, the PDE2A mRNA levels are the highest in the caudate lobe, nucleus accumbens, cortex (frontal, parietal and temporal) and the hippocampus, and are at least 10-fold lower expression in other brain regions (Non-Patent Document 15). This suggests that PDE2A may control intraneuronal cAMP and cGMP levels in areas that are important for learning and memory formation.
Inhibition of PDE2A results in increased cAMP and cGMP levels that could improve cognitive function. In both cortical neurons and hippocampal slices, a PDE2A inhibitor potently increased cGMP concentrations in the presence of guanylate cyclase activators and also increased cAMP concentrations in the presence of forskolin (Non-Patent Document 16). The PDE2A inhibitor was also found to potently increase the induction of long-term potentiation (LTP) in hippocampal slices in response to a weak tetanizing stimulus. This effect on LTP in slices suggests that PDE2A inhibition has positive effects on learning and memory in vivo (Non-Patent Document 16). In fact, the same PDE2A inhibitor increased retention on both novel object and social recognition tasks in rats, and improved object memory and object recognition task in 3-, 12-, and 24-month old rats. It also attenuated the extradimensional (ED) shift deficit on extradimensional-intradimensional (ED/ID) cognitive task in subchronic PCP-treated rats (Non-Patent Document 16-18). These results suggest that PDE2A inhibition could facilitate learning and memory processes through potentiation of cAMP and cGMP-regulated signaling cascades.
Increased cGMP levels by PDE2A inhibition could also influence anxiety and stress-related events. PDE2A inhibitors decreased oxidative stress and induced the expression of NADPH oxidase subunits in oxidative stress inducer-treated mice. It improved anxiety-like behavior in elevated plus maze, open-field, and hole-board tests through the NADPH oxidase pathway (Non-Patent Document 19). In addition, PDE2A inhibitors also produced anxiolytic effects on behavior in non-stressed mice in the elevated plus-maze and hole-board tests (Non-Patent Document 20). PDE2A may be a novel pharmacological target for treatment of not only cognitive deficit, but also anxiety in neuropsychiatric and neurodegenerative disorders.
These unique distribution and functions in the brain indicate that PDE2A represents an important novel target for the treatment of neuropsychiatric and neurodegenerative disorders, in particular schizophrenia and Alzheimer's disease.
Patent Document 1 discloses

However, the structure of the present invention is different from that of the above-mentioned compound.